U.S. patent application number 10/384949 was filed with the patent office on 2003-11-27 for tuner.
Invention is credited to Cowley, Nicholas Paul, Mudd, Mark Stephen John.
Application Number | 20030220088 10/384949 |
Document ID | / |
Family ID | 9932727 |
Filed Date | 2003-11-27 |
United States Patent
Application |
20030220088 |
Kind Code |
A1 |
Cowley, Nicholas Paul ; et
al. |
November 27, 2003 |
Tuner
Abstract
A tuner is provided for converting any selected channel in an
input band to zero intermediate frequency. The tuner input is
connected to the inputs of a plurality of input stages. Each input
stage comprises tracking filters having a passband which is tunable
across the tuning range of the input stage, where the tuning ranges
of the stages form a contiguous or overlapping set so as to provide
tuning across the whole of the input band. Only one of the input
stages is enabled at a time with the disabled stages being
depowered. A frequency changer converts the selected channel from
the enabled input stage to substantially zero intermediate
frequency.
Inventors: |
Cowley, Nicholas Paul;
(Wiltshire, GB) ; Mudd, Mark Stephen John;
(Wiltshire, GB) |
Correspondence
Address: |
Mark D. Saralino
Renner, Otto, Boisselle & Sklar, LLP
Nineteenth Floor
1621 Euclid Avenue
Cleveland
OH
44115
US
|
Family ID: |
9932727 |
Appl. No.: |
10/384949 |
Filed: |
March 10, 2003 |
Current U.S.
Class: |
455/292 ;
455/307; 455/324 |
Current CPC
Class: |
H03J 3/08 20130101; H03J
2200/32 20130101; H03J 1/0008 20130101; H03J 5/244 20130101 |
Class at
Publication: |
455/292 ;
455/307; 455/324 |
International
Class: |
H04B 001/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2002 |
GB |
0205698.4 |
Claims
What is claimed is:
1. A tuner for converting any selected one of a plurality of radio
frequency channels to a substantially zero intermediate frequency,
comprising: a radio frequency input; a plurality of input stages
having inputs connected to said radio frequency input, each of said
input stages comprising at least one tracking filter having a
passband which is tunable across a tuning range, said tuning ranges
of said input stages being different from each other and said input
stages being arranged to be enabled one at a time; and a frequency
changer arrangement for converting said selected channel from an
enabled one of said input stages to said substantially zero
intermediate frequency.
2. A tuner as claimed in claim 1, in which said radio frequency
channels contain digitally modulated signals.
3. A tuner as claimed in claim 1, in which adjacent ones of at
least two of said tuning ranges are contiguous with each other.
4. A tuner as claimed in claim 1, in which adjacent ones of at
least two of said tuning ranges overlap each other.
5. A tuner as claimed in claim 1, comprising baseband filtering for
filtering output signals of said frequency changer arrangement.
6. A tuner as claimed in claim 5, in which said baseband filtering
comprises low pass filtering.
7. A tuner as claimed in claim 6, in which said low pass filtering
has a selectable cut-off frequency.
8. A tuner as claimed in any one of claims 5, comprising an
analog/digital converter, said baseband filtering having an output
connected to said analog/digital converter.
9. A tuner as claimed in claim 1, in which each of said input
stages comprises a band limit filter for attenuating signals
outside said tuning range.
10. A tuner as claimed in claim 1, in which each of said input
stages comprises an automatic gain control circuit.
11. A tuner as claimed in claim 1, in which said at least one
tracking filter of each of said input stages comprises a bandpass
filter.
12. A tuner as claimed in claim 11, in which each of said bandpass
filters comprises an image reject filter.
13. A tuner as claimed in claim 11, in which each of the bandpass
filters has a passband substantially equal to the width of each of
the channels.
14. A tuner as claimed in claim 1, in which said at least one
tracking filter of each of said input stages comprises a local
oscillator reject filter.
15. A tuner as claimed in claim 1, in which each of said input
stages is arranged to be depowered when disabled.
16. A tuner as claimed in claim 1, in which said input stages have
outputs and said frequency changer arrangement comprises a single
frequency changer having an input connected to said outputs of said
input stages.
17. A tuner as claimed in claim 16, in which said single frequency
changer is a quadrature frequency changer.
18. A tuner as claimed in any one of claims 15, in which said
single frequency changer has a mixer and a local oscillator whose
frequency is above a frequency band containing said channels.
19. A tuner as claimed in claim 18, in which said single frequency
changer has a programmable frequency divider between said mixer and
said local oscillator.
20. A tuner as claimed in claim 18, in which said local oscillator
comprises a voltage controlled oscillator for receiving a control
voltage and each of said tracking filters has a voltage-dependent
component for receiving said control voltage.
21. A tuner as claimed in claim 18, in which said local oscillator
comprises a voltage controlled oscillator for receiving a control
voltage and each of said tracking filters has a voltage-dependent
component for receiving a sum of said control voltage and a
respective offset voltage.
22. A tuner as claimed in claim 1, in which said frequency changer
arrangement comprises a respective frequency changer in each of
said input channels.
23. A tuner as claimed in claim 22, in which each of said
respective frequency changers is a quadrature frequency
changer.
24. A tuner as claimed in claim 22, in which each of said
respective frequency changers has a mixer, said tuner comprising at
least one local oscillator whose frequency is above a frequency
band containing said channels.
25. A tuner as claimed in claim 24, comprising at least one
programmable frequency divider between said mixers and said at
least one local oscillator.
26. A tuner as claimed in claim 24, in which said at least one
local oscillator comprises a voltage controlled oscillator for
receiving a control voltage and each of said tracking filters has a
voltage-dependent component for receiving a sum of said control
voltage and a respective offset voltage.
27. A tuner as claimed claim 1, in which each of said tracking
filters has a digital/analog converter and a voltage-dependent
component connected to said digital/analog converter, said
digital/analog converters having inputs and said tuner comprising a
memory connected to said converter inputs, said memory containing a
look-up table and being arranged to be addressed in accordance with
a channel selection.
28. A tuner as claimed in claim 1, in which each of said tracking
filters comprises a frequency determining component, said tuner
comprising an alignment controller for varying a value of said
frequency-determining component of each of said tracking filters so
as substantially to maximise an amplitude of a signal within said
tuner resulting from a reference signal supplied to said radio
frequency input.
Description
TECHNICAL FIELD
[0001] The present invention relates to a tuner, for example for
use as a digital terrestrial tuner or other multimedia digital
tuner.
BACKGROUND
[0002] FIG. 1 of the accompanying drawings illustrates a known type
of digital terrestrial tuner of the single conversion
superheterodyne type. The tuner has an input 1 for receiving a
radio frequency signal connected to the inputs of three input
stages 2, 3 and 4. The input stages are substantially identical in
their structure and function so that only the input stage 4 will be
described in detail. The input stages 2, 3 and 4 are used for
reception of bands V1, V3 and U, respectively. The three bands
covered by the input stages are contiguous or overlap with each
other and in a typical example cover the range 50 to 860 MHz.
[0003] The input stage 4 comprises an input band limit filter 5
which has fixed characteristics and which passes the band U while
attenuating signals outside this band. The output of the filter 5
is supplied to a first tracking filter 6, which is a single tuned
notch filter for suppressing the local oscillator frequency without
substantially attenuating a channel which has been selected for
reception. The filter includes a variable capacitance diode 7 for
controlling the resonant frequency.
[0004] The output of the filter 6 is supplied to an automatic gain
control (AGC) circuit 8 whose output is supplied to a second
tracking filter 9. The filter 9 is a double tuned bandpass filter
whose centre frequency is controlled by variable capacitance diodes
10 and 11. The filter 9 acts as an image reject tracking filter
whose passband is centred substantially on the channel selected for
reception and has a bandwidth sufficient to pass the selected
channel.
[0005] The output of the filter 9 is supplied to a mixer 12 forming
part of a frequency changer which also comprises a voltage
controlled oscillator 13 tuned by a variable capacitance diode 14.
The frequency of oscillation of the oscillator 13 is controlled by
a phase locked loop (PLL) frequency synthesiser 15 which receives
the oscillator signal and supplies a tuning voltage to the diode
14. The synthesiser 15 is controlled by a controller (not shown)
such that the frequency of the oscillator 13 is substantially equal
to the radio frequency of the selected channel plus the
intermediate frequency (IF).
[0006] Although the synthesiser 15 has been shown within the input
stage 4, in practice a single synthesiser is provided for
controlling the VCOs of all of the input stages 2 to 4. The tuning
voltage is also supplied to the variable capacitance diodes 7, 10
and 11 so that the filters 6 and 9 track the frequency of the VCO
13 but with their resonant frequencies appropriately offset from
the VCO frequency so as to provide image rejection for each
selected channel and suppression of the oscillator output so as to
prevent re-radiation via the tuner input 1.
[0007] The outputs of all of the input stages 2 to 4 are connected
to an amplifier 16, which amplifies the IF signal from the
currently operating input stage and provides impedance matching to
a surface acoustic wave filter (SAWF) 17. The filter 17 is of fixed
bandpass type with its centre frequency at the intermediate
frequency, which is typically 36 MHz. The output of the amplifier
16 is also supplied to a level detection circuit 18, which monitors
the signal amplitude at the output of the amplifier 16 and supplies
a control signal to the AGC circuit 8 so as to maximise the level
of the selected channel while minimising distortion from undesired
interfering signals, which may be of greater amplitude than the
selected channel.
[0008] The output of the filter 17 is supplied to another AGC
circuit or driver 20, which amplifies the IF signal and provides
impedance matching between the filter 17 and a further similar SAWF
19. The output of the driver 20 is also connected to a further
level detecting circuit 21 so as to optimise the signal level at
the input of the filter 19. The output of the filter 19 is supplied
to another AGC circuit 22 whose output supplies the IF output
signal of the tuner, for example to a digital demodulator. The
demodulator (not shown) demodulates the selected channel in
accordance with the modulation standard for the channel and also
supplies an AGC control signal to the circuit 22 so as to optimise
the signal level supplied to the demodulator.
[0009] In a typical application, the input stage 4 receives UHF
signals between 470 and 860 MHz so that the tuning range of the VCO
13 for an IF of 36 MHz is from 506 to 896 MHz. Each channel in a
broadband signal supplied to the tuner input 1 has a bandwidth
which is nominally 8 MHz. The passband of the input stage 4 defined
by the tracking filters should preferably be equal to the channel
bandwidth but, in practice, is generally 2 to 3 times the channel
bandwidth and varies as the filters are tuned across the band.
Outside the filter passband, the attenuation provided by the
filters increases with distances from the passband but, in
practice, only limited attenuation of adjacent channels may be
provided by the input stage filtering.
[0010] A disadvantage of this type of known tuner is that the
output IF requires conversion from analog to digital before or
within the following demodulator. Digitisation at the standard IF
is demanding on analog/digital converter (ADC) performance. Also,
this type of tuner requires the use of relatively large and
expensive surface acoustic wave filters in order to provide a
signal of adequate quality for correct demodulation. Although it
would be desirable to implement the tuner as a single integrated
circuit, surface acoustic wave filters of adequate quality cannot
be implemented within monolithic integrated circuits.
[0011] Although it is theoretically possible to embody the whole of
the tuner except the surface acoustic wave filters as a single
integrated circuit, in practice this is difficult or impossible. In
particular, difficulties are caused by the finite amount of
isolation which may be achieved around an integrated circuit
package. For example, the insertion loss of a typical surface
acoustic wave filter is 20 dB. The inter-filter amplifier 20
typically has a gain of 10 dB so that the gain through the filters
is -30 dB. The leakage path of signals around an integrated circuit
package is typically about -45 dB so that the ratio of the required
signal to the leakage signal is only about 15 dB. The leakage
signal is phase-shifted with respect to the direct signal and so
appears as an interference signal which makes acceptable
demodulation of the selected channel difficult or impossible.
SUMMARY
[0012] According to the invention, there is provided a tuner for
converting any selected one of a plurality of radio frequency
channels to substantially zero intermediate frequency, comprising a
radio frequency input connected to the inputs of a plurality of
input stages, each of which comprises at least one tracking filter
having a passband which is tunable across a tuning range, the
tuning ranges of the input stages being different from each other
and the input stages being arranged to be enabled one at a time;
and a frequency changer arrangement for converting the selected
channel from the enabled one of the input stages to substantially
zero intermediate frequency.
[0013] The radio frequency channels may contain digitally modulated
signals.
[0014] Adjacent ones of at least two of the tuning ranges may be
contiguous or may overlap with each other.
[0015] The tuner may comprise base band filtering for filtering
output signals of the frequency changer arrangement. The baseband
filtering may comprise low pass filtering. The low pass filtering
may have a selectable cut-off frequency. The output of the baseband
filtering may be connected to an analog/digital converter.
[0016] Each of the input stages may comprise a band limit filter
for attenuating signals outside the tuning range.
[0017] Each of the input stages may comprise an automatic gain
control circuit.
[0018] The at least one tracking filter of each of the input stages
may comprise a bandpass filter. Each of the bandpass filters may
comprise an image reject filter. Each of the bandpass filters may
have a passband substantially equal to the width of each of the
channels.
[0019] The at least one tracking filter of each of the input stages
may comprise a local oscillator reject filter, such as a notch
filter.
[0020] Each of the input stages may be arranged to be depowered
when disabled.
[0021] The frequency changer arrangement may comprise a single
frequency changer having an input connected to the output of the
input stages. As an alternative, the frequency changer arrangement
may comprise a respective frequency changer in each of the input
stages. The or each frequency changer may be a quadrature frequency
changer.
[0022] The or each frequency changer may have a mixer and a local
oscillator whose frequency is above the frequency band containing
the channels. The or each frequency changer may have a programmable
frequency divider between the mixer and the local oscillator. The
or each local oscillator may comprise a voltage controlled
oscillator for receiving a control voltage and each of the tracking
filters may have a voltage-dependent component for receiving the
control voltage. As an alternative, the or each local oscillator
may comprise a voltage controlled oscillator for receiving a
control voltage and each of the tracking filters may have a
voltage-dependent component for receiving the sum of the control
voltage and a respective offset voltage.
[0023] Each of the tracking filters may have a voltage-dependent
component connected to a digital/analog converter whose input is
connected to a memory containing a look-up table and arranged to be
addressed in accordance with channel selection.
[0024] The tuner may comprise an alignment controller for varying
the value of a frequency-determining component of each of the
tracking filters so as substantially to maximise the amplitude of a
signal within the tuner resulting from a reference signal supplied
to the radio frequency input.
[0025] It is thus possible to provide a tuner which avoids the need
for external filtering while providing an acceptable performance,
for example with digitally encoded signals. It is possible to
embody such a tuner as a single monolithically integrated circuit
to allow a reduction in size and cost. Relatively low power
consumption can be achieved because the use of tracking filtering
means that active stages such as mixers do not need to have a high
quiescent current which would otherwise be necessary in order to
provide acceptable performance when receiving higher signal levels
of broadband signals. The use of zero intermediate frequency
conversion also reduces the performance requirements of
analog/digital conversion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a block circuit diagram of a known type of
tuner;
[0027] FIG. 2 is a block schematic diagram of a tuner constituting
a first embodiment of the invention;
[0028] FIG. 3 is a block circuit diagram of a tuner constituting a
second embodiment of the invention;
[0029] FIG. 4 is a block circuit diagram of a tuner constituting a
third embodiment of the invention;
[0030] FIG. 5 is a block circuit diagram of a local oscillator
arrangement for use in the tuners shown in FIGS. 2 to 4; and
[0031] FIG. 6 is a block schematic diagram illustrating an
arrangement for providing local oscillator and tracking filter
control which may be used in place of the arrangements shown in the
tuners of FIGS. 2 to 4.
[0032] Like reference numerals refer to like parts throughout the
drawings.
DETAILED DESCRIPTION
[0033] The tuner shown in FIG. 2 is similar to that shown in FIG. 1
in that it comprises three input stages 2 to 4, a mixer 12 and an
oscillator 13 controlled by a frequency synthesiser 15, and an IF
AGC arrangement 22. Each of the input stages 2 to 4 comprises a
band limit filter, single tuned tracking filter, AGC circuit
controlled by the level detector 18 and double tuned tracking
filter as shown in FIG. 1 and operating as described with reference
to FIG. 1. However, the input stages do not contain a frequency
changer.
[0034] The outputs of the input stages are connected to a single
frequency changer which comprises a quadrature mixer 12 and a
quadrature oscillator 13. In practice, the quadrature mixer 12 may
comprise two individual mixers which receive their input signals
from the input stages 2 to 4 and the oscillator 13 may produce
in-phase and quadrature output signals to the respective individual
mixers. A controller 25 controls the operation of the tuner and, in
particular, the operation of the frequency synthesiser 15 for
selecting a channel for reception. The frequency synthesiser
controls the quadrature local oscillator 13 in the well-known way
and in addition supplies control signals to the input stages 2 to
4. The band limit filtering in the input stages 2 to 4 divides the
input band into three sub-bands and the control signals from the
frequency synthesiser 15 enable the input stage for the sub-band
containing the channel which has been selected for reception by the
controller 25. In particular, the frequency synthesiser 15 disables
the other input stages by depowering them. The frequency
synthesiser also supplies the appropriate control voltages to the
tracking filters in the enabled input stage so as to pass the
selected channel and so as to suppress the local oscillator
frequency to prevent reradiation from the tuner input 1.
[0035] The frequency changer converts the selected channel to zero
intermediate frequency and supplies baseband I and Q output
signals. The local oscillator signals supplied to the individual
mixers of the arrangement 12 have a frequency substantially equal
to the centre frequency of the selected channel and the passband of
the tracking filtering in the enabled input stage is similarly
centred on the centre frequency of the selected channel. As
described hereinafter, the local oscillator 13 comprises a voltage
controlled oscillator and a programmable frequency divider and the
local oscillator reject filtering provides a notch response at the
frequency of the voltage controlled oscillator so that the selected
channel is substantially unaffected by the local oscillator reject
filtering.
[0036] The quadrature baseband outputs of the mixer arrangement 12
are supplied to a filtering arrangement 26 which comprises a
baseband filter 27 for the in-phase signal I and a baseband filter
28 for the quadrature signal Q. Each of the baseband filters 27 and
28 comprises a low pass filter whose cut-off or turnover frequency
can be selected in accordance with the channel bandwidth. For
example, in the case of the DVB standard system where the channel
bandwidth is 8 MHz, the baseband filters 27 and 28 may have a
cut-off frequency of 4.1 MHz and may be sixth order Chebychev low
pass filters.
[0037] The outputs of the filters are supplied to the AGC circuit
22, which comprises individual AGC stages 29 and 30 having an AGC
input 31 for receiving a control signal from a digital demodulator
(not shown). The output signals of the stages 29 and 30 are
supplied to I and Q outputs 32 and 33 of the tuner for connection
to the digital demodulator, which includes analog/digital
conversion and performs all the functions associated with
demodulation and error correction in the digital domain in
accordance with the digital modulation standard of the selected
channel. The cut-off frequency of the baseband filtering may be
selected by a control signal from the demodulator or from the
controller 25 so as to be appropriate to the channel bandwidth.
[0038] FIG. 3 illustrates a tuner which differs principally from
that shown in FIG. 2 in that the single frequency changer
arrangement common to all of the input stages 2 to 4 is replaced by
an individual frequency changer at the output of each of the input
stages 2 to 4. The circuit arrangements are identical for all of
the input stages and only that of the stage 4 is shown in
detail.
[0039] FIG. 3 shows a voltage controlled oscillator 13 connected
via a quadrature generator 23 to the quadrature mixing arrangement
12.
[0040] In the tuner of FIG. 3, the tuning voltage generated by the
synthesiser 15 is supplied not only to the oscillation
frequency-determining variable capacitance diode 14 of the VCO 13
but also to the variable capacitance diodes 7, 10 and 11 of the
tracking filters 6 and 9. During manufacture of the tuner, the
tracking filters 6 and 9 are adjusted so that they track the
frequency of the local oscillator signal supplied to the mixing
arrangement 12 over the tuning range of the input stage. For
example, the input stage 2 may have a tuning range from 50 to 160
MHz, the input stage 3 may have a tuning range from 160 to 470 MHz
and the input stage 4 may have a tuning range from 470 to 860 MHz.
However, because the frequency changer is of the zero intermediate
frequency type, there is no need for adjustment of the filters to
offset their centre frequencies from the local oscillator frequency
when the same voltage is applied, as is required in the known tuner
shown in FIG. 1.
[0041] In the tuner shown in FIG. 3, the frequency of the local
oscillator signal supplied to the mixer 12 is at the centre of the
channel which has been selected for reception. The filter 6
therefore cannot be a notch filter tuned to the local oscillator
frequency (as in the case of the tuner shown in FIG. 1) but is a
single-tuned bandpass filter centred on the frequency of the
selected channel so as to provide further attenuation of
out-of-band interfering signals. Alternatively, the filter 6 may
comprise a tracking filter, such as a notch filter, tuned to
provide attenuation of harmonics of the local oscillator frequency
or to attenuate other interfering signals so as to prevent such
signals being radiated from the tuner input 1 or from being
otherwise received from the tuner or converted to base band by the
mixer 12.
[0042] The tuner shown in FIG. 4 differs from that shown in FIG. 3
in the way the control voltages for the variable capacitance diodes
7, 10, 11 and 14 are generated. The control voltage for the diode
14 of the VCO 13 is generated in the usual way by the synthesiser
15 in accordance with the channel selection by the controller 25.
However, the control voltages for the diodes 7, 10 and 11 are
generated by a triple digital/analog converter (DAC) controlled by
the controller 25. The output of each DAC is connected to the
respective diode so that the diode voltages can be controlled
independently. The digital inputs of the DACs are supplied by the
controller 25, for example from a memory which contains look-up
tables representing a function of the diode voltage against the
channel selection. The look-up table may be created and stored in
the controller 25 during an alignment phase of the manufacture of
the tuner which ensures that the tracking filters are correctly
aligned with the VCO 13. Each look-up table may have an entry for
each channel. Alternatively, there may be look-up table entries for
a limited number of the channels spread across the input band and
the diode control voltages for channels without entries may be
obtained by interpolation so as to reduce the memory requirements
in the controller 25.
[0043] In an alternative arrangement, the circuit of FIG. 4 may be
modified such that the control voltage from the synthesiser 15 is
supplied to the cathodes of all of the variable capacitance diodes
7, 10, 11 and 14 with the control voltages from the triple DAC 35
supplied as offset voltages referenced to ground to the anodes of
the diodes 7, 10 and 11.
[0044] FIG. 5 illustrates a local oscillator arrangement which may
be used in any of the tuners shown in FIGS. 2 to 4. The arrangement
comprises a plurality (two in the example shown) of voltage
controlled oscillators 13a, 13b having oscillation frequency
determining circuits 40a, 40b in the form of inductance/capacitance
(LC) tank circuits. The tank circuits include voltage controlled
capacitance characteristics for tuning the resonant frequency and
hence the frequency of oscillation. In order to enhance the tuning
range, each oscillator may have an additional element, shown as
capacitors 41a, 41b, which may be switched in parallel or series
with the resonant network of the tank circuit by electronic
bandswitches 42a and 42b.
[0045] The VCO outputs are connected to a multiplexer 43 for
selecting which oscillator is to be active. For example, the
multiplexer 43 may comprise a summer and the or each non-selected
oscillator may be depowered. The output of the multiplexer 43 is
supplied to a programmable divider comprising a first divider 44
for selectively dividing the frequency of its input signal by 2 or
3 and a second divider 45 for selectively dividing the frequency of
its input signal by 8, 4, 2 or 1. The total division ratio is
controlled by a ratio select circuit 46. The output of the second
divider 45 is supplied to the circuit 23, which performs a further
divide by 2 and forms the quadrature local oscillator output
signals I and Q.
[0046] The following table illustrates the frequency ranges which
may be obtained from the local oscillator arrangement shown in FIG.
5.
1 prescaler ratio quad ratio minimum frequency maximum frequency 2
2 500 1000 3 2 533 667 4 2 250 500 6 2 167 333 8 2 125 250 12 2 83
167 16 2 62.5 125 24 2 42 83 VCO minimum frequency 2000 MHz VCO
maximum frequency 4000 MHz
[0047] In this example, the frequency range of signals supplied to
the programmable divider is from 2000 to 4000 MHz. This range may
be provided by a single VCO, a single VCO with bandswitching, or
two or more VCOs each with or without bandswitching. The first
column headed "prescaler ratio" illustrates the different frequency
division ratios which may be provided by the dividers 44 and 45.
The second column headed "quad ratio" illustrates the frequency
division ratio by the circuit 23 which is always present. For each
prescaler ratio, the minimum and maximum frequencies are
illustrated in the next two columns of the table.
[0048] With such an arrangement, the VCO frequency is always above
the highest frequency of the input band to the tuner and the local
oscillator reject filter 6 in each of the input stages 2 to 4
tracks the VCO frequency so as to suppress leakage of the VCO
signal to the tuner input, from which it may cause interference
with other equipment, for example by reradiation from an aerial
connected to the tuner input or directly through a cable
distribution network to which the tuner input is connected. Also,
leakage of the VCO output at such relatively high frequencies
substantially avoids interference with reception of the selected
channel.
[0049] FIG. 6 illustrates another technique for generating the
control voltages for the variable capacitance diodes. The VCO 13 is
controlled by the synthesiser 15 in the normal way, the synthesiser
receiving control signals via a data bus 50 from the controller 25.
The data bus is also connected to DACs 51, 52 and 53, whose outputs
are connected to first inputs of summers 54, 55 and 56
respectively. The second inputs of the summers 54 to 56 receive the
control voltage Vtune from the synthesiser 15 used to control the
oscillation frequency Flo of the VCO 13.
[0050] As described hereinbefore, the controller 25 controls the
synthesiser 15 such that the output frequency Flo of the VCO 13 is
equal to the channel centre frequency of the channel which is
selected for reception. The synthesiser 15 supplies a control
voltage Vtune which is such as to phase-lock the VCO to the
required frequency and this control voltage is supplied to the
summers 54 to 56. The DACs 51 to 53 receive inputs from the
controller 25 for generating offset voltages so that the output
voltages of the summers 54 to 56 supplied to the variable
capacitance diodes 7, 10 and 11 are such that the tracking filters
correctly track the VCO frequency.
[0051] As described hereinbefore, the digital signals supplied to
the DACs 51 to 53 may be derived from a look-up table generated
during a manufacturing alignment operation. Alternatively, the
controller 25 may be arranged to perform an alignment operation,
for example each time the tuner is powered or each time the channel
selection is changed. For example, a reference signal may be
injected into the input of the tuner and the controller may supply
digital codes so as to sweep the output voltage of each DAC in turn
to locate the correct voltage for the respective diode, for example
the voltage giving maximum signal amplitude after the frequency
changer. Such an arrangement has the advantage that the effect of
component values drifting with time may be substantially eliminated
as the tuner is periodically realigned and this compensates for any
component value changes. However, the performing of such a
realigning operation takes time.
[0052] An alternative technique is a combination of the above
described techniques such that an initial value is supplied from a
look-up table and the alignment operation is then used to correct
for any residual alignment errors. Because the initial value should
be reasonably close to the desired value, the time required for the
alignment operation can be substantially reduced.
* * * * *